The present invention relates to the field of image sensors.
Focal plane arrays for sensing light images have been traditionally constructed such that each tiny picture element or pixel detector was as close to its adjacent detector as practicable in order to sample as much of the input scene as possible. This is known as a high fill factor, where the fill factor is defined as the ratio of detector area over the total area of the array. Past emphasis in design was focused on creating greater than ninety percent fill factor arrays.
Recently there has been interest in creating arrays that perform certain low level operations on-chip such a placing transistors or cooling devices on the chip between the image sensing detectors. One approach for achieving this goal is to use a low fill-factor device, i.e., increase the separation between the detectors to accommodate placement of the transistors or cooling devices between the detectors and place a light collecting device such as a micro-lens array in front of the detector array. There are three traditional approaches to collecting light for use with a focal plane array. The most simple is to use a one-to-one imaging system to image the scene upon the array. The disadvantage of this approach is that any part of the image that does not impinge on the detector will be lost. A second approach is to magnify the image. Although this approach is viable, it requires that the magnified image be limited to the size of the total array. Thus the size of the input image is restricted to 1/M times the size of the array where M is the magnification factor.
A third approach utilizes micro-lens arrays. The problem with this approach is that if the lens arrays are diffractive in nature, then higher orders add to the cross talk. In addition, even if they are not diffractive, off-axis rays could be focussed on adjacent detectors, again increasing the cross talk between adjacent detectors.